A variety of systems are known in color recording technology. For example, systems such as electrophotographic systems, ink jet systems and heat sensitive transfer systems are already in practical use. Of these, the heat sensitive transfer system is in wide use since it is considered more advantageous than the other systems because the maintenance and operation of related equipment are easy and this equipment and the associated consumables are economical. However, a heat sensitive transfer system involves the setting in position of two sheets, an ink sheet and a transferred sheet, and therefore necessitates a variety of adjustments relating to material and equipment if high image quality is to be achieved. In particular, forming multicolor images involves successive transfer of a plurality of ink sheets and in this case therefore it is essential to take especial care to avoid color fringing. Consequently, there have been calls for non-transfer type color heat sensitive recording materials permitting direct thermal recording using just a single sheet of recording paper.
Thermal color development material as typified by heat sensitive paper is known as non-transfer type heat sensitive recording materials. However, although there have been attempts to make such a material into a multicolor recording material, no full-color heat sensitive recording material is known as yet.
To produce full-color heat sensitive recording materials, it is preferable to use materials with the characteristic that compounds per se go irreversibly from being colorless to being colored on heating, or conversely go from being colored to being colorless on heating. The compounds disclosed in the specification of WO No. 86-07,312 are known as the former type of compounds which change from being colorless to being colored on heating, but the disclosed compounds have the drawback that the changes in hue are slow and that quite a high temperature is needed. Compounds which are decolored through the action of alkalis are known as the latter type of compounds which are changed from being colored to being colorless, from, e.g., U.S. Pat. No. 4,258,118, but no examples of compounds which are changed to colorless simply through heating alone have been reported.
It is the object of the present invention to provide dyes which change rapidly from being colored to being colorless on heating. The invention also has as its object to provide a heat sensitive recording material comprising such dyes.